1. Field of the Invention
This invention relates to combustors used in gas turbine engines, and more particularly, to film cooling gas turbine engine combustor liners.
2. Description of Related Art
Combustor liners are generally used in the combustion section of gas turbine engines located between the compressor and turbine sections of the engine and in the exhaust sections of afterburning aircraft gas turbine engines. Combustors generally include an exterior casing and an interior combustor. Fuel is burned in the interior of the combustor producing a hot gas usually at an intensely high temperature such as 3,000.degree. F. or even higher. A heat shield or combustor liner is provided to prevent this intense heat from damaging the combustion section and rest of the engine.
Some aircraft gas turbine engines, particularly ones that are capable of supersonic flight such as military fighters and bombers, have afterburners or augmenters located in the exhaust section of the engine. Heat shields or liners are also provided for afterburner engines to prevent the intense combustion heat from damaging the surrounding casing of the exhaust section or other parts of the engine and aircraft.
A more detailed discussion of the related art may be found in related U.S. Patent Application Ser. No. 07/614,418 entitled "GAS TURBINE ENGINE MULTI-HOLE FILM COOLED COMBUSTOR LINER AND METHOD OF MANUFACTURE", invented by Wakeman et al., filed Nov. 15, 1990, and U.S. Pat. application Ser. No. 07/614,368 entitled "COMBUSTOR LINER WITH CIRCUMFERENTIALLY ANGLED FILM COOLING HOLES", by Napoli, filed Nov. 15, 1990, both applications assigned to the same assignee as in the present application. Both of these references are incorporated herein by reference.
Engine designers have long sought to incorporate light weight easy to manufacture combustor liners capable of withstanding the temperatures and pressure differentials found in gas turbine engine combustors. To that end the invention described in the Wakeman reference provides a single wall, preferably sheet metal, annular combustor liner having multi-hole film cooling holes that are disposed through the wall of the liner at sharp downstream angles. The multi-hole film cooling holes are essentially cylindrical and spaced closely together to form at least one continuous pattern designed to provide film cooling over the length of the liner. The present invention provides multi-hole film cooling holes that are cylindrical and have a diameter of about 20 mils with a nominal tolerance of about .+-.2 mils, are spaced closely together about 6 1/2 to 7 1/2 hole diameters apart, have a downstream angle of 20 degrees with a nominal tolerance of about .div.1 degree. Axially adjacent holes are circumferentially offset by half the angle between circumferentially adjacent holes to further enhance the evenness of the cooling film injection points. The Wakeman reference further discloses an embodiment wherein the liner may be corrugated so as to form a wavy wall that is designed to prevent buckling and is particularly useful for aircraft gas turbine engine afterburners.
A phenomenon that occurs on combustor liners both in the main combustion section and in the afterburner combustion section is swirl, wherein swirled patterns of higher thermal degradation areas are formed on the liner in areas where relative hot zones exist. The patterns generally coincide with the swirl of the combustor flow induced by swirlers in the fuel nozzles that are used to promote better combustion and in the exhaust section by turbine nozzles. Liners subject to these uneven heating patterns are difficult and costly to cool.
Combustor liner cooling uses a significant percentage of cooling air that is taken from the compressor for the main combustion section and usually from the fan section for afterburner combustor liners. This is costly air in terms of fuel and power consumption and therefore aircraft engine designers are always seeking means for reducing the amount of cooling air required to cool afterburner liners. Sufficient cooling air is conventionally provided in order to cool the hottest areas caused by swirl and some cooling air is wasted in supplying cooling air to relatively cool zones.